1. Field of the Invention
The present invention generally relates to smear compensation circuits for eliminating a smear component which is contained in and peculiar to an output of a solid state imager and, more particularly, to a smear compensation circuit for compensating for a smear of an output from camera apparatus such as a video camera using a solid state image pickup device or the like.
2. Description of the Prior Art
The occurrence of a color aliasing signal known as a smear is enumerated as one of phenomena peculiar to solid state image pickup devices. The smear component occurs when a light is directly introduced into a vertical transfer section or when charges produced within a semiconductor substrate are expanded by the diffusion and mixed into the vertical transfer section. The smear component tends to occur particularly when the cameraman takes a picture of an object having high brightness, which causes the quality of a reproduced image to be deteriorated.
As conventional smear compensation circuits for compensating for the smear component inherent in the solid state image pickup devices, there is known a smear compensation circuit in which a smear component in the vertical blanking period is written in a memory and a smear component read out from the memory during the video period is subtracted from a video signal to thereby compensate for the smear component because only the smear component exists during the vertical blanking period and the smear component is superposed on the video signal during the video period. Japanese Laid-Open Patent Publication No. 63-173474, for example, describes this conventional smear compensation circuit.
However, the conventional smear compensation circuit, which uses a memory of a limited bit capacity in order to reduce the capacity of the memory, could not fulfil both a quantization noise (data length per bit) and a dynamic range (maximum compensation range). That is, if the former is satisfied, then a compensation signal is clipped when there are too much smear component and the smear components cannot be fully compensated for. If the latter is satisfied, then a vertical distortion appears due to a quantization noise, which causes the quality of image to be deteriorated.
This disadvantage encountered with the prior art will be described with reference to FIG. 1 in which a block input is 8 bits and a memory capacity is 6 bits.
When higher-order 2 bits are neglected as shown at (a) in FIG. 1, then a so-called round-off noise does not occur but the smear component of 2 bits is not compensated. Further, when lower-order 2 bits are neglected at (b) in FIG. 1, then the smear component is fully compensated but the round-off noise occurs. That is, in the above two cases, since 8 bits are stored in the memory having the capacity of 6 bits, the round-off noise can neither be removed nor the smear component can be fully compensated for.